1. Field of the Invention
The present invention relates to digital data communication systems and methods. Specifically, the present invention relates to the equalization of digital data communication signals in order to compensate for transmission impairments on the line. Even more specifically, the present invention relates to the equalization of digital data communication signals wherein one transceiver of the two is resource limited. In a specific application, this invention relates to load sharing between the digital signal processor of a customer premises modem and the digital signal processor of a central office line card by use of a Tomlinson-Harashima precoder.
2. Description of the Related Art
Since shortly after the invention of the digital electronic computer, it has been necessary to transfer data from one computer to another. Early methods, such as punch cards, were quite slow and primitive by today's standards, and it has long been apparent that it would be advantageous to be able to transmit data more quickly and efficiently. Although computer data is, generally speaking, digital, methods were quickly developed to transmit such data through infrastructure that was designed to transmit analog data. Such analog infrastructure included telephone lines (via modems), radio transmissions, and the like. The ability to transfer digital data has improved tremendously over the last few decades. In fact, digital data communications have in many ways surpassed analog data communications, and data that has traditionally been transmitted through analog communications is now being transmitted digitally, for instance, mobile phones and digital television. Despite the tremendous advances in digital data communications, the potential benefit of further advances has become even more apparent. Demand to transmit data through digital data communications has kept up with the extraordinary advances in digital data communications. Such increases in demand are largely attributable to increased use of the Internet and of mobile phones. Thus, a tremendous need exists for faster and more accurate digital data communications.
Digital data is typically transmitted as a series of symbols. The speed of digital data communication can be increased by either reducing the amount of time provided for each symbol, or increasing the amount of data (bits) contained in each symbol. Which symbol is being transmitted is typically indicated by varying the amplitude, phase angle, frequency or some combination thereof, of the signal. However, as the time provided for each symbol decreases, and the number of different symbols that may be transmitted increases, it becomes more and more difficult to distinguish one symbol from another. The characteristics of the transmission line, or other medium, typically effect the signal. For instance, some frequencies may be attenuated more than others, and some frequencies may travel faster through the transmission line than others. These transmission medium characteristics that effect the signal are called channel distortions, and are a major factor in limiting the speed at which digital data can be communicated. However, it is possible to compensate for channel distortions to some degree by selectively filtering or amplifying different frequency ranges. Such selective compensation of frequency ranges is called equalization, and is effective at increasing the rate at which digital data can be transmitted.
Digital communication systems may employ a number of initialization, training, and adaptive learning protocols that are designed to equalize the channel distortions, optimize the data transmission speed, reduce transmission errors, and improve the quality of the received signal. Precoding has been used to provide near-optimal equalization in a number of applications, such as in V.34 modems and HDSL2 transceivers. Generally the precoding is applied in a symmetric fashion, where both receivers in a communication link generate precoding filters during a startup sequence, and transfer the results to the remote transmitter for insertion in a precoder arrangement. In some cases a linear pre-equalizer or pre-emphasis filter is transferred as well to effectively perform all of the channel equalization in the transmitter.
The current generation of pulse code modulation (PCM) modems, i.e., modem systems compliant with ITU-T Recommendation V.90, perform an initial training procedure to adaptively adjust the equalizer structure resident at the client-side analog modem (APCM). In addition, an echo cancellor architecture resident at the server-side digital modem (DPCM) may be adaptively trained during an initialization period such that the echo channel associated with the DPCM is adequately emulated. V.90 modem systems perform an initial two-point training procedure in the downstream direction during which one constellation signal point (based on a particular μ-law or A-law level) is transmitted as a sequence having positive and negative signs. The DPCM transmits the two-point training sequence to the APCM, and the APCM analyzes the received signal to determine the channel characteristics and to adjust its equalizers. In the upstream direction, a 4 or 16 point QAM modulated signal sequence is transmitted to the DPCM receiver, which generates a precoder filter based on the received sequence and transmits the precoder taps to the APCM, which in turn inserts that precoder filter into its precoder. Digital Data Communication in general, and Equalization in particular, is discussed in Lee & Messerschmitt, DIGITAL, COMMUNICATION, (2d ed. 1996), the contents of which are incorporated herein by reference.
3. Overview of the Invention
An equalization arrangement using preceding can be taken advantage of in situations where resources are limited in one of the two transceivers, or indeed, where resources are limited for most receivers in a multi-point communication system. Relieving the receiver of the equalization task generally reduces its processing load considerably. However, the effort of generating the precoder and possibly pre-equalizer filters still remains. In the following, systems and methods are shown where calculation of these filters can be performed even under strict resource limitations, enabling high-performance equalization with minimal processing load on one end of the connection
One significant application of this method is in communicating with central-office line cards. The increasing of data transmission rates beyond rates offered by V.90 is possible by making the line card an active participant in the data connection from an analog-connected modem to a digitally connected central site. This could be done using V.90 symbol rates and encoding methods, or by redesigning the modulation methods and possibly extending the symbol rates beyond V.90. Line-cards could also provide a constant low-rate on-hook channel to achieve an “always-on” connection. Present here is a modulation and equalization method that preferably can accommodate all these scenarios in a comprehensive fashion, preferably obtaining near-optimal performance given the resource restraints in the line card. The modulation in the opposite direction from the line-card to the modem is not a concern. That direction may employ conventional preceding techniques or indeed, in the interest of saving line-card resources, not use preceding at all. It will be expected that a communication link does exist.
This invention presents systems and methods of modulation and equalization for scenarios where one of two transceivers is resource limited. An exemplary precoding method itself is a form of Tomlinson preceding combined preferably with pre-equalization that is primarily phase equalization. Several systems and methods of obtaining precoder and pre-equalizer taps are presented, one based on channel-estimation techniques, and another based on shortened equalization. The former method allows greater flexibility in meeting resource requirements, but may become unnecessarily cumbersome overall for low-rate applications. The latter method allows a simple and effective equalization scheme for low data rates, but may require more effort in obtaining more optimal equalization at higher rates. Different configurations may favor one or the other, indeed some applications may combine the two in some form.